The invention relates to a method for inspecting laminated iron cores of electrical machines for interlamination shorts, in which the laminated iron core is magnetized by means of an auxiliary winding and the iron surface is scanned by means of a measurement coil arrangement with a downstream measurement device.
The invention also relates to an apparatus for carrying out the method, and to a computer program for data evaluation.
Laminated iron cores, in particular of electrical machines, are frequently inspected for interlamination shorts during production and in the course of maintenance work during operation, using the measurement method of stator laminate ring excitation with rated induction. This method which indicates the effect of interlamination short currents in the form of local temperature differences, demands a powerful voltage source which can be regulated, and field windings with large cross sections (so-called hot spot method using, for example, infrared cameras). In the case of stators with built-in winding bars, this inspection can identify only those fault points (short circuit between a number of laminates) on the tooth surface, and not the laminate faults in the slot base or on the slot sides. This method identifies only interlamination shorts with a specific contact resistance and resultant local temperature differences, and hence not all interlamination short points. The temperature increase on its own is not sufficient for quantitative assessment of the interlamination short point. However, quantitative assessment of these fault points is worthwhile since, in the end, the volt-amperes in a fault point are a critical criterion for determining whether such a region is able to initiate so-called xe2x80x9ccore meltingxe2x80x9d, that is to say melting occurring during operation as a result of excessive heating at the fault point.
U.S. Pat. No. 5,321,362 describes a so-called low-induction method for determining the short-circuit currents at interlaminate contacts, in which the laminated core is first of all weakly magnetized using a coil placed around the laminated stator core, and the stray field on the laminated core is then measured using a measurement coil arrangement. This method has the advantage that it does not require such high induction currents and, furthermore, it is possible to localize not only fault points that are located on the tooth surface. In this case, it is also possible to identify fault points which are not heated at that time during the method mentioned above, and the measurement process can be carried out with the rotor installed. U.S. Pat. No. 5,321,362 is in this case based exclusively on a current measurement, and therefore does not make it possible to draw any reliable conclusion with regard to the volt-amperes flowing in the fault point.
Secondly, CH 676526 A5 describes a method in which fault points can be localized not only on the tooth surface but also in the slot base and on the slot sides, and in a quantitative manner, as well. This is once again a low-induction method, but in this case it is not simply the current that is measured, but a reference measurement is also proposed, which allows the volt-amperes flowing in a fault point to be determined directly and quantitatively, by appropriate back-calculation.
One fundamental problem in the context of such low-induction methods is the fact that they frequently do not allow unique localization of fault points, since differences in the behavior of the magnetic iron core (for example resulting from cooling slots) can also lead to signals which, with these methods, may possibly also be interpreted as fault points.
One object of the present invention is therefore to propose a method for inspecting laminated iron cores of electrical machines for interlamination shorts, in which method the laminated iron core is magnetized by means of an auxiliary winding and the iron surface is scanned by means of at least one measurement coil arrangement with a downstream measurement device, and which allows fault points, that is to say any interlamination shorts which may be present, to be localized as uniquely as possible.
The object is achieved in that both the phase and the amplitude of the electrical signal which is induced in the at least one measurement coil arrangement are used in combined form to localize interlamination shorts. Thus, in contrast to the methods according to the prior art, not only the amplitude or not only the phase is used to localize the interlamination shorts, but the phase and amplitude information is used in a combined manner to identify them.
This is because, surprisingly, it has been found that the amplitude and phase when combined, that is to say effectively considering the vector of the current induced in the measurement coil arrangement, allow unique localization of the interlamination shorts, and in a quantitative manner as well.
According to a first preferred embodiment of the present invention, an interlamination short is identified in that a real part of the induced electrical signal occurs which is high in comparison to the intact regions. Specifically, it has been found that such a high real part actually uniquely characterizes those fault points, that is to say regions, at which individual laminates are connected to one another via a short circuit. In this case, this high real part can preferably be determined in particular by means of a polar representation.
The direction which in this context is referred to as the xe2x80x9creal partxe2x80x9d is, so to speak, a xe2x80x9cpronounced directionxe2x80x9d and is defined, for example, by the embodiment described further below, in which a reference is fed in using a calibration current coil (loop) with an appropriate width and with an appropriate laminate current.
According to a further preferred embodiment of the invention, the method can be combined with a gradient measurement, that is to say with a measurement which also allows unique and quantifiable localization of the fault point in the radial direction with respect to the stator. This is done by arranging at least one further measurement coil arrangement radially at a distance, which further measurement coil arrangement is electrically isolated from a first measurement coil arrangement but is mechanically connected to the fist measurement coil arrangement, and by using both the phase and the amplitude of the at least two electrical signals, which are induced in the at least two measurement coil arrangements, to localize interlamination shorts, in particular to localize them in the radial direction.
Another preferred embodiment is characterized in that the induced electrical signals are compared with signals which correspond to the interlamination short currents, with a loop with an appropriate width being mounted on the surface of the laminated iron core as a reference for this purpose, and being fed with an appropriate interlamination short current. This method allows quantification of the fault points, that is to say, in particular, it allows determination of the volt-amperes which effectively flow in a fault point. It is thus possible to detect the actual fault points with reference to any possible xe2x80x9ccore meltingxe2x80x9d.
The auxiliary winding for magnetization of the laminated iron cores can, according to a further preferred embodiment, be provided such that the laminated iron core is magnetized in particular via the rotor shaft from a high-power supply source which is connected between the two machine sides, in particular on the insulated shaft. In this case the laminated iron core is preferably magnetized in the range from 0.03 to 0.7 Tesla, in particular approximately 0.1 Tesla, with a voltage of 220/110 volts at 50/60 Hz.
In a further preferred embodiment of the present invention, the at least one measurement coil arrangement, or in the case of a gradient measurement the at least two measurement coil arrangements, is or are shifted on the laminated iron core in the axial direction. In this case, the induced signals are determined, in particular in the form of a fingerprint, as a function of the position of the measurement coil arrangement, with regions with critical interlamination shorts being determined, in particular, via the real part and being distinguished from regions with noncritical interlamination shorts, and with the critical regions being marked for immediate rectification, and the regions which are (still) noncritical (at that time) being documented for later analysis.
Furthermore, the present invention relates to an apparatus for carrying out a method as described above, comprising first means for determining the position of the measurement coil arrangement on the laminated iron core, comprising second means for detecting the amplitude and the phase, in particular by means of a phase detector, of the electrical voltage which is induced in the measurement coil arrangement, and also comprising a data processing unit, which analyzes the measured data and, in particular, is able to localize critical interlamination shorts and to determine the volt-amperes flowing in them, by comparison with a reference. Normally, the position of the measurement coil arrangement can in this case be detected using the first means, and the measured value can be detected using the second means. This information is supplied to the data processing system, thus making it possible to obtain an overall image of the state of the stator. The first means may in this case be designed, for example, in the form of a measurement unit relating to the movement position of a robot carriage to which the measurement coil arrangement or arrangements is or are fitted.
Further preferred embodiments of the method according to the invention and of the apparatus according to the invention are described in the dependent patent claims.
In addition, the present invention relates to a computer program for carrying out the analysis in a data processing unit, as is used in the apparatus mentioned above. The computer program is characterized in that the data, that is to say the phase and amplitude of the electrical voltage which is induced in the measurement coil arrangement, is changed to a vector representation for example in the form of a polar diagram, preferably after having been adjusted using a reference, and fault points are automatically distinguished from noncritical regions, in that only high amplitude values which at the same time have a high real part are identified as being critical.
This is done, for example, by determining fault points on the basis of the real part of the signal exceeding a predetermined limit value, with the limit value preferably being the mean value of all the data over the entire length of the laminated iron core. This determination process may in this case be carried out either automatically, or else, so to speak, manually by the user, that is to say the user assesses the polar diagram to decide which regions should be included in the fault table, and which should not.
Further preferred embodiments of the computer program according to the invention are described in the dependent patent claims.